Many modern systems utilize power in a pulsed delivery mode, repeatedly supplying energy to an energy storage element, typically charging a storage capacitor, and rapidly delivering the stored energy into a load. For example, medical lasers operate by delivering optical energy in a rapid-fire series of short laser bursts. These bursts are generated by rapidly discharging an energy storage element such as a capacitor into a flash lamp. The capacitor is repeatedly charged by a power supply unit (PSU) and discharged into the flash lamp (load). Other similar pulsed-mode power supply applications include laser diode arrays, strobe lights, such as those used for stop-motion photography, and pulsed beacons, such as those used on broadcasting towers, tall buildings, aircraft, etc.
Energy storage capacitors are used to store energy for pulsed load applications. It is undesirable for the power supply to directly provide the pulsed load current, as this would require a larger, heavier supply and draw excessive transient currents from the source of power. Drawing power in such a manner is disliked by utility companies because the high RMS currents result in conduction (I.sup.2 R) losses, requiring heavier wire, switches and connectors to avoid overheating. A good power factor requires the current waveform to avoid large peaks--that is, to follow the voltage waveform. Regulations are arising which require a good power factor. The RMS current capability of wiring circuit breakers and outlets is limited, and this invention permits a higher output power from a given power source current capacity.
Traditionally, laser power supply manufacturers have measured power factor (PF) into steady-state load conditions, but in reality this does not reflect normal operation. To provide the high peak currents required by a typical flash lamp or laser diode loads, a power supply unit (PSU) charges an energy storage element up to a predetermined energy level, after which the stored energy can be rapidly discharged into a load.
Using an storage capacitor as an example, a power supply unit (PSU) charges the capacitor up to a predetermined level. The energy (E) stored in the capacitor is given by the equation: EQU E=1/2CV.sup.2
where: PA1 "An electric power storage unit consisting of a series combination of capacitor cells. The storage unit is electrically charged with a constant charging current from a charger with a simple structure. If necessary, the simple structure remotely controls the limit voltages of the capacitor cells and varies the capacity and output power of the storage unit according to use conditions. When the terminal voltages reach reference voltages, parallel charging control units bypass the charging current. The parallel charging control units comprise shunt regulators connected in parallel with their respective capacitor cells and acting to bypass the charging current, coupling circuits for connection with a signal source and reference voltage control circuits. The reference voltage control the circuits establish reference voltages according to the output signals from the coupling circuits, compare the terminal voltages of the capacitor cells and control the shunt regulators according to the results of the comparisons. The control units can be digitized using digital signals." (Abstract)
C is the capacitance and PA2 V is the charged voltage).
Typically, when the pulsed load (e.g., flash lamp or laser diode array) is switched on (i.e., when the energy storage element is discharged into the load), the storage capacitor acts as a source of high peak current to drive the load, and discharges rapidly. After each pulsed discharge, the PSU's output voltage regulation control loop senses that the capacitor voltage is low and turns on the PSU fully. The PSU operates at its maximum power until the capacitor reaches a predetermined energy storage level. The PSU then stops charging and "idles" to maintain regulation (full charge), replacing losses, until the next discharge pulse. Factors affecting the available recharge time are line voltage tolerance, pulse energy demanded, pulse repetition rate demanded, capacitance value tolerance and temperature variation tolerances. As a consequence, it is normal for the PSU to draw high current, then idle for a period of time, resulting in an unfavorable (low) PF.
Prior-art PSU designs have generally been directed to charging the energy storage element fast enough to support the maximum output pulse rate (minimum pulse period, or time between subsequent pulses). In doing so, however, input power factor has generally been ignored.
Most capacitor charging power supplies (PSUs) provide a constant current output. The maximum output current is limited by the output circuit and rectifier current ratings. At the start of a charging cycle the output voltage is low, so the instantaneous delivered power (the product of voltage and current) is low. At the end of the charging cycle, the delivered power is very high, typically twice the average charging power. For pulse-forming networks where the capacitor is discharged during each output pulse, this charging ramp of power creates a similar ramp of current draw from the power source, again resulting in poor power factor.
Both active and passive techniques are known in the prior art for improving (correcting) the power factor of electrical devices.
Passive power factor correction techniques smooth the current during each cycle, and since they operate at line frequencies (50-60 cycles per second), they require large, heavy, expensive components and are limited to 0.8 or 0.9 PF. (A "perfect" power factor is 1.0) Further, their fixed values do not readily accommodate the wide range of pulse repetition rates and duty cycles that may be required of a pulsed load.
Active power factor correction techniques attempt to match the cycle-by-cycle current drawn through the input voltage cycle and are therefore incapable of correcting changes in demand occurring over several cycles or fractions of a cycle without large bulk storage capacitors, which increase the size weight, cost and safety hazards.
A practical unity (1.0) power factor is achieved only in the particular case of a steady state load demand where the input current waveform matches the input voltage waveform over a number of cycles at a level that approximates the average power drawn. Another way to express this is that even if each cycle is separately PF-corrected, fluctuations in the power of each individual cycle will cause a greater RMS current value and heating effect than a steady average value of power per cycle.
The challenge in achieving a high power factor is particularly difficult for "pulse mode" power supplies, since in many cases the pulse repetition rate and pulse energy are not fixed, but are user selectable.
A typical example of a prior art method and apparatus for controlling charging of an electrical power storage unit can be found in U.S. Pat. No. 5,726,552, incorporated in its entirety by reference herein. As disclosed therein,